/**********************************************************************
Copyright (c) 2016 Andy Jefferson and others. All rights reserved.
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.

Contributors:
     ...
 **********************************************************************/
package org.datanucleus.util;

import java.io.IOException;
import java.io.Serializable;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;
import java.util.AbstractCollection;
import java.util.AbstractMap;
import java.util.AbstractSet;
import java.util.Collection;
import java.util.EnumSet;
import java.util.Enumeration;
import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.concurrent.locks.ReentrantLock;

/**
 * An advanced hash table supporting configurable garbage collection semantics of keys and values, optional
 * referential-equality, full concurrency of retrievals, and adjustable expected concurrency for updates.
 * <p>
 * This table is designed around specific advanced use-cases. If there is any doubt whether this table is for
 * you, you most likely should be using {@link java.util.concurrent.ConcurrentHashMap} instead.
 * <p>
 * This table supports strong, weak, and soft keys and values. By default keys are weak, and values are
 * strong. Such a configuration offers similar behavior to {@link java.util.WeakHashMap}, entries of this
 * table are periodically removed once their corresponding keys are no longer referenced outside of this
 * table. In other words, this table will not prevent a key from being discarded by the garbage collector.
 * Once a key has been discarded by the collector, the corresponding entry is no longer visible to this table;
 * however, the entry may occupy space until a future table operation decides to reclaim it. For this reason,
 * summary functions such as <tt>size</tt> and <tt>isEmpty</tt> might return a value greater than the observed
 * number of entries. In order to support a high level of concurrency, stale entries are only reclaimed during
 * blocking (usually mutating) operations.
 * <p>
 * Enabling soft keys allows entries in this table to remain until their space is absolutely needed by the
 * garbage collector. This is unlike weak keys which can be reclaimed as soon as they are no longer referenced
 * by a normal strong reference. The primary use case for soft keys is a cache, which ideally occupies memory
 * that is not in use for as long as possible.
 * <p>
 * By default, values are held using a normal strong reference. This provides the commonly desired guarantee
 * that a value will always have at least the same life-span as it's key. For this reason, care should be
 * taken to ensure that a value never refers, either directly or indirectly, to its key, thereby preventing
 * reclamation. If this is unavoidable, then it is recommended to use the same reference type in use for the
 * key. However, it should be noted that non-strong values may disappear beforeQuery their corresponding key.
 * <p>
 * While this table does allow the use of both strong keys and values, it is recommended to use
 * {@link java.util.concurrent.ConcurrentHashMap} for such a configuration, since it is optimized for that
 * case.
 * <p>
 * Just like {@link java.util.concurrent.ConcurrentHashMap}, this class obeys the same functional
 * specification as {@link java.util.Hashtable}, and includes versions of methods corresponding to each method
 * of <tt>Hashtable</tt>. However, even though all operations are thread-safe, retrieval operations do
 * <em>not</em> entail locking, and there is <em>not</em> any support for locking the entire table in a way
 * that prevents all access. This class is fully interoperable with <tt>Hashtable</tt> in programs that rely
 * on its thread safety but not on its synchronization details.
 * <p>
 * Retrieval operations (including <tt>get</tt>) generally do not block, so may overlap with update operations
 * (including <tt>put</tt> and <tt>remove</tt>). Retrievals reflect the results of the most recently
 * <em>completed</em> update operations holding upon their onset. For aggregate operations such as
 * <tt>putAll</tt> and <tt>clear</tt>, concurrent retrievals may reflect insertion or removal of only some
 * entries. Similarly, Iterators and Enumerations return elements reflecting the state of the hash table at
 * some point at or since the creation of the iterator/enumeration. They do <em>not</em> throw
 * {@link java.util.ConcurrentModificationException}. However, iterators are designed to be used by only one
 * thread at a time.
 * <p>
 * The allowed concurrency among update operations is guided by the optional <tt>concurrencyLevel</tt>
 * constructor argument (default <tt>16</tt>), which is used as a hint for internal sizing. The table is
 * internally partitioned to try to permit the indicated number of concurrent updates without contention.
 * Because placement in hash tables is essentially random, the actual concurrency will vary. Ideally, you
 * should choose a value to accommodate as many threads as will ever concurrently modify the table. Using a
 * significantly higher value than you need can waste space and time, and a significantly lower value can lead
 * to thread contention. But overestimates and underestimates within an order of magnitude do not usually have
 * much noticeable impact. A value of one is appropriate when it is known that only one thread will modify and
 * all others will only read. Also, resizing this or any other kind of hash table is a relatively slow
 * operation, so, when possible, it is a good idea to provide estimates of expected table sizes in constructors.
 * <p>
 * This class and its views and iterators implement all of the <em>optional</em> methods of the {@link Map}
 * and {@link Iterator} interfaces.
 * <p>
 * Like {@link java.util.Hashtable} but unlike {@link java.util.HashMap}, this class does <em>not</em> allow
 * <tt>null</tt> to be used as a key or value.
 * <p>
 * This class is a member of the <a href="{@docRoot}/../technotes/guides/collections/index.html"> Java Collections Framework</a>.
 * 
 * <b>Written by Doug Lea with assistance from members of JCP JSR-166 Expert Group and released to the public domain, as explained at http://creativecommons.org/licenses/publicdomain</b>
 * 
 * 
 * @param <K> the type of keys maintained by this map
 * @param <V> the type of mapped values 
 */
public class ConcurrentReferenceHashMap<K, V> extends AbstractMap<K, V> implements java.util.concurrent.ConcurrentMap<K, V>, Serializable
{
    private static final long serialVersionUID = 7249069246763182397L;

    /*
     * The basic strategy is to subdivide the table among Segments, each of which itself is a concurrently readable hash table.
     */

    /**
     * An option specifying which Java reference type should be used to refer to a key and/or value.
     */
    public static enum ReferenceType
    {
        /** Indicates a normal Java strong reference should be used */
        STRONG,
        /** Indicates a {@link WeakReference} should be used */
        WEAK,
        /** Indicates a {@link SoftReference} should be used */
        SOFT
    }

    public static enum Option
    {
        /**
         * Indicates that referential-equality (== instead of .equals()) should be used when locating keys.
         * This offers similar behavior to {@link java.util.IdentityHashMap}
         */
        IDENTITY_COMPARISONS
    }

    /* ---------------- Constants -------------- */

    static final ReferenceType DEFAULT_KEY_TYPE = ReferenceType.STRONG;
    static final ReferenceType DEFAULT_VALUE_TYPE = ReferenceType.WEAK;

    /**
     * The default initial capacity for this table, used when not otherwise specified in a constructor.
     */
    static final int DEFAULT_INITIAL_CAPACITY = 16;

    /**
     * The default load factor for this table, used when not otherwise specified in a constructor.
     */
    static final float DEFAULT_LOAD_FACTOR = 0.75f;

    /**
     * The default concurrency level for this table, used when not otherwise specified in a constructor.
     */
    static final int DEFAULT_CONCURRENCY_LEVEL = 16;

    /**
     * The maximum capacity, used if a higher value is implicitly specified by either of the constructors with
     * arguments. MUST be a power of two <= 1<<30 to ensure that entries are indexable using ints.
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * The maximum number of segments to allow; used to bound constructor arguments.
     */
    static final int MAX_SEGMENTS = 1 << 16; // slightly conservative

    /**
     * Number of unsynchronized retries in size and containsValue methods beforeQuery resorting to locking.
     * This is used to avoid unbounded retries if tables undergo continuous modification which would make it
     * impossible to obtain an accurate result.
     */
    static final int RETRIES_BEFORE_LOCK = 2;

    /* ---------------- Fields -------------- */

    /**
     * Mask value for indexing into segments. The upper bits of a key's hash code are used to choose the
     * segment.
     */
    final int segmentMask;

    /**
     * Shift value for indexing within segments.
     */
    final int segmentShift;

    /**
     * The segments, each of which is a specialized hash table
     */
    final Segment<K, V>[] segments;

    boolean identityComparisons;

    transient Set<K> keySet;

    transient Set<Map.Entry<K, V>> entrySet;

    transient Collection<V> values;

    /* ---------------- Small Utilities -------------- */

    /**
     * Applies a supplemental hash function to a given hashCode, which defends against poor quality hash
     * functions. This is critical because ConcurrentReferenceHashMap uses power-of-two length hash tables,
     * that otherwise encounter collisions for hashCodes that do not differ in lower or upper bits.
     */
    private static int hash(int h)
    {
        // Spread bits to regularize both segment and index locations,
        // using variant of single-word Wang/Jenkins hash.
        h += (h << 15) ^ 0xffffcd7d;
        h ^= (h >>> 10);
        h += (h << 3);
        h ^= (h >>> 6);
        h += (h << 2) + (h << 14);
        return h ^ (h >>> 16);
    }

    /**
     * Returns the segment that should be used for key with given hash
     * @param hash the hash code for the key
     * @return the segment
     */
    final Segment<K, V> segmentFor(int hash)
    {
        return segments[(hash >>> segmentShift) & segmentMask];
    }

    private int hashOf(Object key)
    {
        return hash(
            identityComparisons ? System.identityHashCode(key) : key.hashCode());
    }

    /* ---------------- Inner Classes -------------- */

    static interface KeyReference
    {
        int keyHash();

        Object keyRef();
    }

    /**
     * A weak-key reference which stores the key hash needed for reclamation.
     */
    static final class WeakKeyReference<K> extends WeakReference<K> implements KeyReference
    {
        final int hash;

        WeakKeyReference(K key, int hash, ReferenceQueue<Object> refQueue)
        {
            super(key, refQueue);
            this.hash = hash;
        }

        @Override
        public final int keyHash()
        {
            return hash;
        }

        @Override
        public final Object keyRef()
        {
            return this;
        }
    }

    /**
     * A soft-key reference which stores the key hash needed for reclamation.
     */
    static final class SoftKeyReference<K> extends SoftReference<K> implements KeyReference
    {
        final int hash;

        SoftKeyReference(K key, int hash, ReferenceQueue<Object> refQueue)
        {
            super(key, refQueue);
            this.hash = hash;
        }

        @Override
        public final int keyHash()
        {
            return hash;
        }

        @Override
        public final Object keyRef()
        {
            return this;
        }
    }

    static final class WeakValueReference<V> extends WeakReference<V> implements KeyReference
    {
        final Object keyRef;

        final int hash;

        WeakValueReference(V value, Object keyRef, int hash, ReferenceQueue<Object> refQueue)
        {
            super(value, refQueue);
            this.keyRef = keyRef;
            this.hash = hash;
        }

        @Override
        public final int keyHash()
        {
            return hash;
        }

        @Override
        public final Object keyRef()
        {
            return keyRef;
        }
    }

    static final class SoftValueReference<V> extends SoftReference<V> implements KeyReference
    {
        final Object keyRef;

        final int hash;

        SoftValueReference(V value, Object keyRef, int hash, ReferenceQueue<Object> refQueue)
        {
            super(value, refQueue);
            this.keyRef = keyRef;
            this.hash = hash;
        }

        @Override
        public final int keyHash()
        {
            return hash;
        }

        @Override
        public final Object keyRef()
        {
            return keyRef;
        }
    }

    /**
     * ConcurrentReferenceHashMap list entry. Note that this is never exported out as a user-visible
     * Map.Entry.
     * <p/>
     * Because the value field is volatile, not final, it is legal wrt the Java Memory Model for an
     * unsynchronized reader to see null instead of initial value when read via a data race. Although a
     * reordering leading to this is not likely to ever actually occur, the Segment.readValueUnderLock method
     * is used as a backup in case a null (pre-initialized) value is ever seen in an unsynchronized access
     * method.
     */
    static final class HashEntry<K, V>
    {
        final Object keyRef;

        final int hash;

        volatile Object valueRef;

        final HashEntry<K, V> next;

        HashEntry(
                K key, int hash, HashEntry<K, V> next, V value,
                ReferenceType keyType, ReferenceType valueType,
                ReferenceQueue<Object> refQueue)
        {
            this.hash = hash;
            this.next = next;
            this.keyRef = newKeyReference(key, keyType, refQueue);
            this.valueRef = newValueReference(value, valueType, refQueue);
        }

        final Object newKeyReference(
                K key, ReferenceType keyType,
                ReferenceQueue<Object> refQueue)
        {
            if (keyType == ReferenceType.WEAK)
            {
                return new WeakKeyReference<K>(key, hash, refQueue);
            }
            if (keyType == ReferenceType.SOFT)
            {
                return new SoftKeyReference<K>(key, hash, refQueue);
            }

            return key;
        }

        final Object newValueReference(
                V value, ReferenceType valueType,
                ReferenceQueue<Object> refQueue)
        {
            if (valueType == ReferenceType.WEAK)
            {
                return new WeakValueReference<V>(value, keyRef, hash, refQueue);
            }
            if (valueType == ReferenceType.SOFT)
            {
                return new SoftValueReference<V>(value, keyRef, hash, refQueue);
            }

            return value;
        }

        final K key()
        {
            if (keyRef instanceof KeyReference)
            {
                return ((Reference<K>) keyRef).get();
            }

            return (K) keyRef;
        }

        final V value()
        {
            return dereferenceValue(valueRef);
        }

        final V dereferenceValue(Object value)
        {
            if (value instanceof KeyReference)
            {
                return ((Reference<V>) value).get();
            }

            return (V) value;
        }

        final void setValue(V value, ReferenceType valueType, ReferenceQueue<Object> refQueue)
        {
            this.valueRef = newValueReference(value, valueType, refQueue);
        }

        static final <K, V> HashEntry<K, V>[] newArray(int i)
        {
            return new HashEntry[i];
        }
    }

    /**
     * Segments are specialized versions of hash tables. This subclasses from ReentrantLock opportunistically,
     * just to simplify some locking and avoid separate construction.
     */
    static final class Segment<K, V> extends ReentrantLock
    {
        /*
         * Segments maintain a table of entry lists that are ALWAYS kept in a consistent state, so can be read
         * without locking. Next fields of nodes are immutable (final). All list additions are performed at
         * the front of each bin. This makes it easy to check changes, and also fast to traverse. When nodes
         * would otherwise be changed, new nodes are created to replace them. This works well for hash tables
         * since the bin lists tend to be short. (The average length is less than two for the default load
         * factor threshold.) Read operations can thus proceed without locking, but rely on selected uses of
         * volatiles to ensure that completed write operations performed by other threads are noticed. For
         * most purposes, the "count" field, tracking the number of elements, serves as that volatile variable
         * ensuring visibility. This is convenient because this field needs to be read in many read operations
         * anyway: - All (unsynchronized) read operations must first read the "count" field, and should not
         * look at table entries if it is 0. - All (synchronized) write operations should write to the "count"
         * field afterQuery structurally changing any bin. The operations must not take any action that could
         * even momentarily cause a concurrent read operation to see inconsistent data. This is made easier by
         * the nature of the read operations in Map. For example, no operation can reveal that the table has
         * grown but the threshold has not yet been updated, so there are no atomicity requirements for this
         * with respect to reads. As a guide, all critical volatile reads and writes to the count field are
         * marked in code comments.
         */

        private static final long serialVersionUID = 2249069246763182397L;

        /**
         * The number of elements in this segment's region.
         */
        transient volatile int count;

        /**
         * Number of updates that alter the size of the table. This is used during bulk-read methods to make
         * sure they see a consistent snapshot: If modCounts change during a traversal of segments computing
         * size or checking containsValue, then we might have an inconsistent view of state so (usually) must
         * retry.
         */
        transient int modCount;

        /**
         * The table is rehashed when its size exceeds this threshold. (The value of this field is always
         * <tt>(int)(capacity *
         * loadFactor)</tt>.)
         */
        transient int threshold;

        /**
         * The per-segment table.
         */
        transient volatile HashEntry<K, V>[] table;

        /**
         * The load factor for the hash table. Even though this value is same for all segments, it is
         * replicated to avoid needing links to outer object.
         * @serial
         */
        final float loadFactor;

        /**
         * The collected weak-key reference queue for this segment. This should be (re)initialized whenever
         * table is assigned,
         */
        transient volatile ReferenceQueue<Object> refQueue;

        final ReferenceType keyType;

        final ReferenceType valueType;

        final boolean identityComparisons;

        Segment(
                int initialCapacity, float lf, ReferenceType keyType,
                ReferenceType valueType, boolean identityComparisons)
        {
            loadFactor = lf;
            this.keyType = keyType;
            this.valueType = valueType;
            this.identityComparisons = identityComparisons;
            setTable(HashEntry.<K, V> newArray(initialCapacity));
        }

        static final <K, V> Segment<K, V>[] newArray(int i)
        {
            return new Segment[i];
        }

        private boolean keyEq(Object src, Object dest)
        {
            return identityComparisons ? src == dest : src.equals(dest);
        }

        /**
         * Sets table to new HashEntry array. Call only while holding lock or in constructor.
         */
        void setTable(HashEntry<K, V>[] newTable)
        {
            threshold = (int) (newTable.length * loadFactor);
            table = newTable;
            refQueue = new ReferenceQueue<Object>();
        }

        /**
         * Returns properly casted first entry of bin for given hash.
         */
        HashEntry<K, V> getFirst(int hash)
        {
            HashEntry<K, V>[] tab = table;
            return tab[hash & (tab.length - 1)];
        }

        HashEntry<K, V> newHashEntry(K key, int hash, HashEntry<K, V> next, V value)
        {
            return new HashEntry<K, V>(key, hash, next, value, keyType, valueType, refQueue);
        }

        /**
         * Reads value field of an entry under lock. Called if value field ever appears to be null. This is
         * possible only if a compiler happens to reorder a HashEntry initialization with its table
         * assignment, which is legal under memory model but is not known to ever occur.
         */
        V readValueUnderLock(HashEntry<K, V> e)
        {
            lock();
            try
            {
                removeStale();
                return e.value();
            }
            finally
            {
                unlock();
            }
        }

        /* Specialized implementations of map methods */

        V get(Object key, int hash)
        {
            if (count != 0)
            { // read-volatile
                HashEntry<K, V> e = getFirst(hash);
                while (e != null)
                {
                    if (e.hash == hash && keyEq(key, e.key()))
                    {
                        Object opaque = e.valueRef;
                        if (opaque != null)
                        {
                            return e.dereferenceValue(opaque);
                        }

                        return readValueUnderLock(e); // recheck
                    }
                    e = e.next;
                }
            }
            return null;
        }

        boolean containsKey(Object key, int hash)
        {
            if (count != 0)
            { // read-volatile
                HashEntry<K, V> e = getFirst(hash);
                while (e != null)
                {
                    if (e.hash == hash && keyEq(key, e.key()))
                    {
                        return true;
                    }
                    e = e.next;
                }
            }
            return false;
        }

        boolean containsValue(Object value)
        {
            if (count != 0)
            { // read-volatile
                HashEntry<K, V>[] tab = table;
                int len = tab.length;
                for (int i = 0; i < len; i++)
                {
                    for (HashEntry<K, V> e = tab[i]; e != null; e = e.next)
                    {
                        Object opaque = e.valueRef;
                        V v;

                        if (opaque == null)
                        {
                            v = readValueUnderLock(e); // recheck
                        }
                        else
                        {
                            v = e.dereferenceValue(opaque);
                        }

                        if (value.equals(v))
                        {
                            return true;
                        }
                    }
                }
            }
            return false;
        }

        boolean replace(K key, int hash, V oldValue, V newValue)
        {
            lock();
            try
            {
                removeStale();
                HashEntry<K, V> e = getFirst(hash);
                while (e != null && (e.hash != hash || !keyEq(key, e.key())))
                {
                    e = e.next;
                }

                boolean replaced = false;
                if (e != null && oldValue.equals(e.value()))
                {
                    replaced = true;
                    e.setValue(newValue, valueType, refQueue);
                }
                return replaced;
            }
            finally
            {
                unlock();
            }
        }

        V replace(K key, int hash, V newValue)
        {
            lock();
            try
            {
                removeStale();
                HashEntry<K, V> e = getFirst(hash);
                while (e != null && (e.hash != hash || !keyEq(key, e.key())))
                {
                    e = e.next;
                }

                V oldValue = null;
                if (e != null)
                {
                    oldValue = e.value();
                    e.setValue(newValue, valueType, refQueue);
                }
                return oldValue;
            }
            finally
            {
                unlock();
            }
        }

        V put(K key, int hash, V value, boolean onlyIfAbsent)
        {
            lock();
            try
            {
                removeStale();
                int c = count;
                if (c++ > threshold)
                {// ensure capacity
                    int reduced = rehash();
                    if (reduced > 0)
                    {
                        // adjust from possible weak cleanups
                        count = (c -= reduced) - 1; // write-volatile
                    }
                }

                HashEntry<K, V>[] tab = table;
                int index = hash & (tab.length - 1);
                HashEntry<K, V> first = tab[index];
                HashEntry<K, V> e = first;
                while (e != null && (e.hash != hash || !keyEq(key, e.key())))
                {
                    e = e.next;
                }

                V oldValue;
                if (e != null)
                {
                    oldValue = e.value();
                    if (!onlyIfAbsent)
                    {
                        e.setValue(value, valueType, refQueue);
                    }
                }
                else
                {
                    oldValue = null;
                    ++modCount;
                    tab[index] = newHashEntry(key, hash, first, value);
                    count = c; // write-volatile
                }
                return oldValue;
            }
            finally
            {
                unlock();
            }
        }

        int rehash()
        {
            HashEntry<K, V>[] oldTable = table;
            int oldCapacity = oldTable.length;
            if (oldCapacity >= MAXIMUM_CAPACITY)
            {
                return 0;
            }

            /*
             * Reclassify nodes in each list to new Map. Because we are using power-of-two expansion, the
             * elements from each bin must either stay at same index, or move with a power of two offset. We
             * eliminate unnecessary node creation by catching cases where old nodes can be reused because
             * their next fields won't change. Statistically, at the default threshold, only about one-sixth
             * of them need cloning when a table doubles. The nodes they replace will be garbage collectable
             * as soon as they are no longer referenced by any reader thread that may be in the midst of
             * traversing table right now.
             */

            HashEntry<K, V>[] newTable = HashEntry.newArray(oldCapacity << 1);
            threshold = (int) (newTable.length * loadFactor);
            int sizeMask = newTable.length - 1;
            int reduce = 0;
            for (int i = 0; i < oldCapacity; i++)
            {
                // We need to guarantee that any existing reads of old Map can
                // proceed. So we cannot yet null out each bin.
                HashEntry<K, V> e = oldTable[i];

                if (e != null)
                {
                    HashEntry<K, V> next = e.next;
                    int idx = e.hash & sizeMask;

                    // Single node on list
                    if (next == null)
                    {
                        newTable[idx] = e;
                    }

                    else
                    {
                        // Reuse trailing consecutive sequence at same slot
                        HashEntry<K, V> lastRun = e;
                        int lastIdx = idx;
                        for (HashEntry<K, V> last = next; last != null; last = last.next)
                        {
                            int k = last.hash & sizeMask;
                            if (k != lastIdx)
                            {
                                lastIdx = k;
                                lastRun = last;
                            }
                        }
                        newTable[lastIdx] = lastRun;
                        // Clone all remaining nodes
                        for (HashEntry<K, V> p = e; p != lastRun; p = p.next)
                        {
                            // Skip GC'd weak refs
                            K key = p.key();
                            if (key == null)
                            {
                                reduce++;
                                continue;
                            }
                            int k = p.hash & sizeMask;
                            HashEntry<K, V> n = newTable[k];
                            newTable[k] = newHashEntry(key, p.hash, n, p.value());
                        }
                    }
                }
            }
            table = newTable;
            return reduce;
        }

        /**
         * Remove; match on key only if value null, else match both.
         */
        V remove(Object key, int hash, Object value, boolean refRemove)
        {
            lock();
            try
            {
                if (!refRemove)
                {
                    removeStale();
                }
                int c = count - 1;
                HashEntry<K, V>[] tab = table;
                int index = hash & (tab.length - 1);
                HashEntry<K, V> first = tab[index];
                HashEntry<K, V> e = first;
                // a ref remove operation compares the Reference instance
                while (e != null && key != e.keyRef && (refRemove || hash != e.hash || !keyEq(key, e.key())))
                {
                    e = e.next;
                }

                V oldValue = null;
                if (e != null)
                {
                    V v = e.value();
                    if (value == null || value.equals(v))
                    {
                        oldValue = v;
                        // All entries following removed node can stay
                        // in list, but all preceding ones need to be
                        // cloned.
                        ++modCount;
                        HashEntry<K, V> newFirst = e.next;
                        for (HashEntry<K, V> p = first; p != e; p = p.next)
                        {
                            K pKey = p.key();
                            if (pKey == null)
                            { // Skip GC'd keys
                                c--;
                                continue;
                            }

                            newFirst = newHashEntry(pKey, p.hash, newFirst, p.value());
                        }
                        tab[index] = newFirst;
                        count = c; // write-volatile
                    }
                }
                return oldValue;
            }
            finally
            {
                unlock();
            }
        }

        final void removeStale()
        {
            KeyReference ref;
            while ((ref = (KeyReference) refQueue.poll()) != null)
            {
                remove(ref.keyRef(), ref.keyHash(), null, true);
            }
        }

        void clear()
        {
            if (count != 0)
            {
                lock();
                try
                {
                    HashEntry<K, V>[] tab = table;
                    for (int i = 0; i < tab.length; i++)
                    {
                        tab[i] = null;
                    }
                    ++modCount;
                    // replace the reference queue to avoid unnecessary stale cleanups
                    refQueue = new ReferenceQueue<Object>();
                    count = 0; // write-volatile
                }
                finally
                {
                    unlock();
                }
            }
        }
    }

    /* ---------------- Public operations -------------- */

    /**
     * Creates a new, empty map with the specified initial capacity, reference types, load factor and
     * concurrency level.
     * <p>
     * Behavioral changing options such as {@link Option#IDENTITY_COMPARISONS} can also be specified.
     * @param initialCapacity the initial capacity. The implementation performs internal sizing to accommodate
     * this many elements.
     * @param loadFactor the load factor threshold, used to control resizing. Resizing may be performed when
     * the average number of elements per bin exceeds this threshold.
     * @param concurrencyLevel the estimated number of concurrently updating threads. The implementation
     * performs internal sizing to try to accommodate this many threads.
     * @param keyType the reference type to use for keys
     * @param valueType the reference type to use for values
     * @param options the behavioral options
     * @throws IllegalArgumentException if the initial capacity is negative or the load factor or
     * concurrencyLevel are nonpositive.
     */
    public ConcurrentReferenceHashMap(
            int initialCapacity,
            float loadFactor, int concurrencyLevel,
            ReferenceType keyType, ReferenceType valueType,
            EnumSet<Option> options)
    {
        if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0)
        {
            throw new IllegalArgumentException();
        }

        if (concurrencyLevel > MAX_SEGMENTS)
        {
            concurrencyLevel = MAX_SEGMENTS;
        }

        // Find power-of-two sizes best matching arguments
        int sshift = 0;
        int ssize = 1;
        while (ssize < concurrencyLevel)
        {
            ++sshift;
            ssize <<= 1;
        }
        segmentShift = 32 - sshift;
        segmentMask = ssize - 1;
        this.segments = Segment.newArray(ssize);

        if (initialCapacity > MAXIMUM_CAPACITY)
        {
            initialCapacity = MAXIMUM_CAPACITY;
        }
        int c = initialCapacity / ssize;
        if (c * ssize < initialCapacity)
        {
            ++c;
        }
        int cap = 1;
        while (cap < c)
        {
            cap <<= 1;
        }

        identityComparisons = options != null && options.contains(Option.IDENTITY_COMPARISONS);

        for (int i = 0; i < this.segments.length; ++i)
        {
            this.segments[i] = new Segment<K, V>(
                    cap, loadFactor,
                    keyType, valueType, identityComparisons);
        }
    }

    /**
     * Creates a new, empty map with the specified initial capacity, load factor and concurrency level.
     * @param initialCapacity the initial capacity. The implementation performs internal sizing to accommodate
     * this many elements.
     * @param loadFactor the load factor threshold, used to control resizing. Resizing may be performed when
     * the average number of elements per bin exceeds this threshold.
     * @param concurrencyLevel the estimated number of concurrently updating threads. The implementation
     * performs internal sizing to try to accommodate this many threads.
     * @throws IllegalArgumentException if the initial capacity is negative or the load factor or
     * concurrencyLevel are nonpositive.
     */
    public ConcurrentReferenceHashMap(
            int initialCapacity,
            float loadFactor, int concurrencyLevel)
    {
        this(
                initialCapacity, loadFactor, concurrencyLevel,
                DEFAULT_KEY_TYPE, DEFAULT_VALUE_TYPE, null);
    }

    /**
     * Creates a new, empty map with the specified initial capacity and load factor and with the default
     * reference types (weak keys, strong values), and concurrencyLevel (16).
     * @param initialCapacity The implementation performs internal sizing to accommodate this many elements.
     * @param loadFactor the load factor threshold, used to control resizing. Resizing may be performed when
     * the average number of elements per bin exceeds this threshold.
     * @throws IllegalArgumentException if the initial capacity of elements is negative or the load factor is
     * nonpositive
     * @since 1.6
     */
    public ConcurrentReferenceHashMap(int initialCapacity, float loadFactor)
    {
        this(initialCapacity, loadFactor, DEFAULT_CONCURRENCY_LEVEL);
    }

    /**
     * Creates a new, empty map with the specified initial capacity, reference types and with default load
     * factor (0.75) and concurrencyLevel (16).
     * @param initialCapacity the initial capacity. The implementation performs internal sizing to accommodate
     * this many elements.
     * @param keyType the reference type to use for keys
     * @param valueType the reference type to use for values
     * @throws IllegalArgumentException if the initial capacity of elements is negative.
     */
    public ConcurrentReferenceHashMap(
            int initialCapacity,
            ReferenceType keyType, ReferenceType valueType)
    {
        this(initialCapacity, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL, keyType, valueType, null);
    }

    /**
     * Creates a new, empty map with the specified initial capacity, and with default reference types (weak
     * keys, strong values), load factor (0.75) and concurrencyLevel (16).
     * @param initialCapacity the initial capacity. The implementation performs internal sizing to accommodate
     * this many elements.
     * @throws IllegalArgumentException if the initial capacity of elements is negative.
     */
    public ConcurrentReferenceHashMap(int initialCapacity)
    {
        this(initialCapacity, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
    }

    /**
     * Creates a new, empty map with a default initial capacity (16), reference types (weak keys, strong
     * values), default load factor (0.75) and concurrencyLevel (16).
     */
    public ConcurrentReferenceHashMap()
    {
        this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
    }

    /**
     * Creates a new map with the same mappings as the given map. The map is created with a capacity of 1.5
     * times the number of mappings in the given map or 16 (whichever is greater), and a default load factor
     * (0.75) and concurrencyLevel (16).
     * @param m the map
     */
    public ConcurrentReferenceHashMap(Map<? extends K, ? extends V> m)
    {
        this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1, DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
        putAll(m);
    }

    /**
     * Returns <tt>true</tt> if this map contains no key-value mappings.
     * @return <tt>true</tt> if this map contains no key-value mappings
     */
    @Override
    public boolean isEmpty()
    {
        final Segment<K, V>[] segments = this.segments;
        /*
         * We keep track of per-segment modCounts to avoid ABA problems in which an element in one segment was
         * added and in another removed during traversal, in which case the table was never actually empty at
         * any point. Note the similar use of modCounts in the size() and containsValue() methods, which are
         * the only other methods also susceptible to ABA problems.
         */
        int[] mc = new int[segments.length];
        int mcsum = 0;
        for (int i = 0; i < segments.length; ++i)
        {
            if (segments[i].count != 0)
            {
                return false;
            }
            mcsum += mc[i] = segments[i].modCount;
        }
        // If mcsum happens to be zero, then we know we got a snapshot
        // beforeQuery any modifications at all were made. This is
        // probably common enough to bother tracking.
        if (mcsum != 0)
        {
            for (int i = 0; i < segments.length; ++i)
            {
                if (segments[i].count != 0 || mc[i] != segments[i].modCount)
                {
                    return false;
                }
            }
        }
        return true;
    }

    /**
     * Returns the number of key-value mappings in this map. If the map contains more than
     * <tt>Integer.MAX_VALUE</tt> elements, returns <tt>Integer.MAX_VALUE</tt>.
     * @return the number of key-value mappings in this map
     */
    @Override
    public int size()
    {
        final Segment<K, V>[] segments = this.segments;
        long sum = 0;
        long check = 0;
        int[] mc = new int[segments.length];
        // Try a few times to get accurate count. On failure due to
        // continuous async changes in table, resort to locking.
        for (int k = 0; k < RETRIES_BEFORE_LOCK; ++k)
        {
            check = 0;
            sum = 0;
            int mcsum = 0;
            for (int i = 0; i < segments.length; ++i)
            {
                sum += segments[i].count;
                mcsum += mc[i] = segments[i].modCount;
            }
            if (mcsum != 0)
            {
                for (int i = 0; i < segments.length; ++i)
                {
                    check += segments[i].count;
                    if (mc[i] != segments[i].modCount)
                    {
                        check = -1; // force retry
                        break;
                    }
                }
            }
            if (check == sum)
            {
                break;
            }
        }
        if (check != sum)
        { // Resort to locking all segments
            sum = 0;
            for (int i = 0; i < segments.length; ++i)
            {
                segments[i].lock();
            }
            for (int i = 0; i < segments.length; ++i)
            {
                sum += segments[i].count;
            }
            for (int i = 0; i < segments.length; ++i)
            {
                segments[i].unlock();
            }
        }
        if (sum > Integer.MAX_VALUE)
        {
            return Integer.MAX_VALUE;
        }
        return (int) sum;
    }

    /**
     * Returns the value to which the specified key is mapped, or {@code null} if this map contains no mapping
     * for the key.
     * <p>
     * More formally, if this map contains a mapping from a key {@code k} to a value {@code v} such that
     * {@code key.equals(k)}, then this method returns {@code v}; otherwise it returns {@code null}. (There
     * can be at most one such mapping.)
     * @throws NullPointerException if the specified key is null
     */
    @Override
    public V get(Object key)
    {
        if (key == null)
        {
            return null;
        }
        int hash = hashOf(key);
        return segmentFor(hash).get(key, hash);
    }

    /**
     * Tests if the specified object is a key in this table.
     * @param key possible key
     * @return <tt>true</tt> if and only if the specified object is a key in this table, as determined by the
     * <tt>equals</tt> method; <tt>false</tt> otherwise.
     * @throws NullPointerException if the specified key is null
     */
    @Override
    public boolean containsKey(Object key)
    {
        if (key == null)
        {
            return false;
        }
        int hash = hashOf(key);
        return segmentFor(hash).containsKey(key, hash);
    }

    /**
     * Returns <tt>true</tt> if this map maps one or more keys to the specified value. Note: This method
     * requires a full internal traversal of the hash table, and so is much slower than method
     * <tt>containsKey</tt>.
     * @param value value whose presence in this map is to be tested
     * @return <tt>true</tt> if this map maps one or more keys to the specified value
     */
    @Override
    public boolean containsValue(Object value)
    {
        if (value == null)
        {
            return false;
        }

        // See explanation of modCount use above

        final Segment<K, V>[] segments = this.segments;
        int[] mc = new int[segments.length];

        // Try a few times without locking
        for (int k = 0; k < RETRIES_BEFORE_LOCK; ++k)
        {
            int mcsum = 0;
            for (int i = 0; i < segments.length; ++i)
            {
                // int c = segments[i].count;
                mcsum += mc[i] = segments[i].modCount;
                if (segments[i].containsValue(value))
                {
                    return true;
                }
            }
            boolean cleanSweep = true;
            if (mcsum != 0)
            {
                for (int i = 0; i < segments.length; ++i)
                {
                    // int c = segments[i].count;
                    if (mc[i] != segments[i].modCount)
                    {
                        cleanSweep = false;
                        break;
                    }
                }
            }
            if (cleanSweep)
            {
                return false;
            }
        }
        // Resort to locking all segments
        for (int i = 0; i < segments.length; ++i)
        {
            segments[i].lock();
        }
        boolean found = false;
        try
        {
            for (int i = 0; i < segments.length; ++i)
            {
                if (segments[i].containsValue(value))
                {
                    found = true;
                    break;
                }
            }
        }
        finally
        {
            for (int i = 0; i < segments.length; ++i)
            {
                segments[i].unlock();
            }
        }
        return found;
    }

    /**
     * Legacy method testing if some key maps into the specified value in this table. This method is identical
     * in functionality to {@link #containsValue}, and exists solely to ensure full compatibility with class
     * {@link java.util.Hashtable}, which supported this method prior to introduction of the Java Collections
     * framework.
     * @param value a value to search for
     * @return <tt>true</tt> if and only if some key maps to the <tt>value</tt> argument in this table as
     * determined by the <tt>equals</tt> method; <tt>false</tt> otherwise
     * @throws NullPointerException if the specified value is null
     */
    public boolean contains(Object value)
    {
        return containsValue(value);
    }

    /**
     * Maps the specified key to the specified value in this table. Neither the key nor the value can be null.
     * <p>
     * The value can be retrieved by calling the <tt>get</tt> method with a key that is equal to the original key.
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with <tt>key</tt>, or <tt>null</tt> if there was no mapping for <tt>key</tt>
     * @throws NullPointerException if the specified key or value is null
     */
    @Override
    public V put(K key, V value)
    {
        if (key == null || value == null)
        {
            return null;
        }
        int hash = hashOf(key);
        return segmentFor(hash).put(key, hash, value, false);
    }

    /**
     * {@inheritDoc}
     * @return the previous value associated with the specified key, or <tt>null</tt> if there was no mapping for the key
     * @throws NullPointerException if the specified key or value is null
     */
    @Override
    public V putIfAbsent(K key, V value)
    {
        if (key == null || value == null)
        {
            return null;
        }
        int hash = hashOf(key);
        return segmentFor(hash).put(key, hash, value, true);
    }

    /**
     * Copies all of the mappings from the specified map to this one. These mappings replace any mappings that
     * this map had for any of the keys currently in the specified map.
     * @param m mappings to be stored in this map
     */
    @Override
    public void putAll(Map<? extends K, ? extends V> m)
    {
        for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
        {
            put(e.getKey(), e.getValue());
        }
    }

    /**
     * Removes the key (and its corresponding value) from this map. This method does nothing if the key is not in the map.
     * @param key the key that needs to be removed
     * @return the previous value associated with <tt>key</tt>, or <tt>null</tt> if there was no mapping for <tt>key</tt>
     * @throws NullPointerException if the specified key is null
     */
    @Override
    public V remove(Object key)
    {
        if (key == null)
        {
            return null;
        }
        int hash = hashOf(key);
        return segmentFor(hash).remove(key, hash, null, false);
    }

    /**
     * {@inheritDoc}
     * @throws NullPointerException if the specified key is null
     */
    @Override
    public boolean remove(Object key, Object value)
    {
        if (key == null || value == null)
        {
            return false;
        }
        int hash = hashOf(key);
        return segmentFor(hash).remove(key, hash, value, false) != null;
    }

    /**
     * {@inheritDoc}
     * @throws NullPointerException if any of the arguments are null
     */
    @Override
    public boolean replace(K key, V oldValue, V newValue)
    {
        if (key == null || oldValue == null || newValue == null)
        {
            throw new NullPointerException();
        }
        int hash = hashOf(key);
        return segmentFor(hash).replace(key, hash, oldValue, newValue);
    }

    /**
     * {@inheritDoc}
     * @return the previous value associated with the specified key, or <tt>null</tt> if there was no mapping for the key
     * @throws NullPointerException if the specified key or value is null
     */
    @Override
    public V replace(K key, V value)
    {
        if (key == null || value == null)
        {
            return null;
        }
        int hash = hashOf(key);
        return segmentFor(hash).replace(key, hash, value);
    }

    /**
     * Removes all of the mappings from this map.
     */
    @Override
    public void clear()
    {
        for (int i = 0; i < segments.length; ++i)
        {
            segments[i].clear();
        }
    }

    /**
     * Removes any stale entries whose keys have been finalized. Use of this method is normally not necessary
     * since stale entries are automatically removed lazily, when blocking operations are required. However,
     * there are some cases where this operation should be performed eagerly, such as cleaning up old
     * references to a ClassLoader in a multi-classloader environment.
     * <p>
     * Note: this method will acquire locks, one at a time, across all segments of this table, so if it is to
     * be used, it should be used sparingly.
     */
    public void purgeStaleEntries()
    {
        for (int i = 0; i < segments.length; ++i)
        {
            segments[i].removeStale();
        }
    }

    /**
     * Returns a {@link Set} view of the keys contained in this map. The set is backed by the map, so changes
     * to the map are reflected in the set, and vice-versa. The set supports element removal, which removes
     * the corresponding mapping from this map, via the <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
     * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt> operations. It does not support the
     * <tt>add</tt> or <tt>addAll</tt> operations.
     * <p>
     * The view's <tt>iterator</tt> is a "weakly consistent" iterator that will never throw
     * {@link java.util.ConcurrentModificationException}, and guarantees to traverse elements as they existed
     * upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications
     * subsequent to construction.
     */
    @Override
    public Set<K> keySet()
    {
        Set<K> ks = keySet;
        return (ks != null) ? ks : (keySet = new KeySet());
    }

    /**
     * Returns a {@link Collection} view of the values contained in this map. The collection is backed by the
     * map, so changes to the map are reflected in the collection, and vice-versa. The collection supports
     * element removal, which removes the corresponding mapping from this map, via the
     * <tt>Iterator.remove</tt>, <tt>Collection.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt>, and
     * <tt>clear</tt> operations. It does not support the <tt>add</tt> or <tt>addAll</tt> operations.
     * <p>
     * The view's <tt>iterator</tt> is a "weakly consistent" iterator that will never throw
     * {@link java.util.ConcurrentModificationException}, and guarantees to traverse elements as they existed
     * upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications
     * subsequent to construction.
     */
    @Override
    public Collection<V> values()
    {
        Collection<V> vs = values;
        return (vs != null) ? vs : (values = new Values());
    }

    /**
     * Returns a {@link Set} view of the mappings contained in this map. The set is backed by the map, so
     * changes to the map are reflected in the set, and vice-versa. The set supports element removal, which
     * removes the corresponding mapping from the map, via the <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
     * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt> operations. It does not support the
     * <tt>add</tt> or <tt>addAll</tt> operations.
     * <p>
     * The view's <tt>iterator</tt> is a "weakly consistent" iterator that will never throw
     * {@link java.util.ConcurrentModificationException}, and guarantees to traverse elements as they existed
     * upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications
     * subsequent to construction.
     */
    @Override
    public Set<Map.Entry<K, V>> entrySet()
    {
        Set<Map.Entry<K, V>> es = entrySet;
        return (es != null) ? es : (entrySet = new EntrySet());
    }

    /**
     * Returns an enumeration of the keys in this table.
     * @return an enumeration of the keys in this table
     * @see #keySet()
     */
    public Enumeration<K> keys()
    {
        return new KeyIterator();
    }

    /**
     * Returns an enumeration of the values in this table.
     * @return an enumeration of the values in this table
     * @see #values()
     */
    public Enumeration<V> elements()
    {
        return new ValueIterator();
    }

    /* ---------------- Iterator Support -------------- */

    abstract class HashIterator
    {
        int nextSegmentIndex;

        int nextTableIndex;

        HashEntry<K, V>[] currentTable;

        HashEntry<K, V> nextEntry;

        HashEntry<K, V> lastReturned;

        K currentKey; // Strong reference to weak key (prevents gc)

        HashIterator()
        {
            nextSegmentIndex = segments.length - 1;
            nextTableIndex = -1;
            advance();
        }

        public boolean hasMoreElements()
        {
            return hasNext();
        }

        final void advance()
        {
            if (nextEntry != null && (nextEntry = nextEntry.next) != null)
            {
                return;
            }

            while (nextTableIndex >= 0)
            {
                if ((nextEntry = currentTable[nextTableIndex--]) != null)
                {
                    return;
                }
            }

            while (nextSegmentIndex >= 0)
            {
                Segment<K, V> seg = segments[nextSegmentIndex--];
                if (seg.count != 0)
                {
                    currentTable = seg.table;
                    for (int j = currentTable.length - 1; j >= 0; --j)
                    {
                        if ((nextEntry = currentTable[j]) != null)
                        {
                            nextTableIndex = j - 1;
                            return;
                        }
                    }
                }
            }
        }

        public boolean hasNext()
        {
            while (nextEntry != null)
            {
                if (nextEntry.key() != null)
                {
                    return true;
                }
                advance();
            }

            return false;
        }

        HashEntry<K, V> nextEntry()
        {
            do
            {
                if (nextEntry == null)
                {
                    throw new NoSuchElementException();
                }

                lastReturned = nextEntry;
                currentKey = lastReturned.key();
                advance();
            }
            while (currentKey == null); // Skip GC'd keys

            return lastReturned;
        }

        public void remove()
        {
            if (lastReturned == null)
            {
                throw new IllegalStateException();
            }
            ConcurrentReferenceHashMap.this.remove(currentKey);
            lastReturned = null;
        }
    }

    final class KeyIterator extends HashIterator implements Iterator<K>, Enumeration<K>
    {
        @Override
        public K next()
        {
            return super.nextEntry().key();
        }

        @Override
        public K nextElement()
        {
            return super.nextEntry().key();
        }
    }

    final class ValueIterator extends HashIterator implements Iterator<V>, Enumeration<V>
    {
        @Override
        public V next()
        {
            return super.nextEntry().value();
        }

        @Override
        public V nextElement()
        {
            return super.nextEntry().value();
        }
    }

    /*
     * This class is needed for JDK5 compatibility.
     */
    static class SimpleEntry<K, V> implements Entry<K, V>, java.io.Serializable
    {
        private static final long serialVersionUID = -8499721149061103585L;

        private final K key;

        private V value;

        public SimpleEntry(K key, V value)
        {
            this.key = key;
            this.value = value;
        }

        public SimpleEntry(Entry<? extends K, ? extends V> entry)
        {
            this.key = entry.getKey();
            this.value = entry.getValue();
        }

        @Override
        public K getKey()
        {
            return key;
        }

        @Override
        public V getValue()
        {
            return value;
        }

        @Override
        public V setValue(V value)
        {
            V oldValue = this.value;
            this.value = value;
            return oldValue;
        }

        @Override
        public boolean equals(Object o)
        {
            if (!(o instanceof Map.Entry))
            {
                return false;
            }
            Map.Entry e = (Map.Entry) o;
            return eq(key, e.getKey()) && eq(value, e.getValue());
        }

        @Override
        public int hashCode()
        {
            return (key == null ? 0 : key.hashCode()) ^ (value == null ? 0 : value.hashCode());
        }

        @Override
        public String toString()
        {
            return key + "=" + value;
        }

        private static boolean eq(Object o1, Object o2)
        {
            return o1 == null ? o2 == null : o1.equals(o2);
        }
    }

    /**
     * Custom Entry class used by EntryIterator.next(), that relays setValue changes to the underlying map.
     */
    final class WriteThroughEntry extends SimpleEntry<K, V>
    {
        private static final long serialVersionUID = -7900634345345313646L;

        WriteThroughEntry(K k, V v)
        {
            super(k, v);
        }

        /**
         * Set our entry's value and write through to the map. The value to return is somewhat arbitrary here.
         * Since a WriteThroughEntry does not necessarily track asynchronous changes, the most recent
         * "previous" value could be different from what we return (or could even have been removed in which
         * case the put will re-establish). We do not and cannot guarantee more.
         */
        @Override
        public V setValue(V value)
        {
            if (value == null)
            {
                throw new NullPointerException();
            }
            V v = super.setValue(value);
            ConcurrentReferenceHashMap.this.put(getKey(), value);
            return v;
        }
    }

    final class EntryIterator
            extends
                HashIterator
            implements
                Iterator<Entry<K, V>>
    {
        @Override
        public Map.Entry<K, V> next()
        {
            HashEntry<K, V> e = super.nextEntry();
            return new WriteThroughEntry(e.key(), e.value());
        }
    }

    final class KeySet extends AbstractSet<K>
    {
        @Override
        public Iterator<K> iterator()
        {
            return new KeyIterator();
        }

        @Override
        public int size()
        {
            return ConcurrentReferenceHashMap.this.size();
        }

        @Override
        public boolean isEmpty()
        {
            return ConcurrentReferenceHashMap.this.isEmpty();
        }

        @Override
        public boolean contains(Object o)
        {
            return ConcurrentReferenceHashMap.this.containsKey(o);
        }

        @Override
        public boolean remove(Object o)
        {
            return ConcurrentReferenceHashMap.this.remove(o) != null;
        }

        @Override
        public void clear()
        {
            ConcurrentReferenceHashMap.this.clear();
        }
    }

    final class Values extends AbstractCollection<V>
    {
        @Override
        public Iterator<V> iterator()
        {
            return new ValueIterator();
        }

        @Override
        public int size()
        {
            return ConcurrentReferenceHashMap.this.size();
        }

        @Override
        public boolean isEmpty()
        {
            return ConcurrentReferenceHashMap.this.isEmpty();
        }

        @Override
        public boolean contains(Object o)
        {
            return ConcurrentReferenceHashMap.this.containsValue(o);
        }

        @Override
        public void clear()
        {
            ConcurrentReferenceHashMap.this.clear();
        }
    }

    final class EntrySet extends AbstractSet<Map.Entry<K, V>>
    {
        @Override
        public Iterator<Map.Entry<K, V>> iterator()
        {
            return new EntryIterator();
        }

        @Override
        public boolean contains(Object o)
        {
            if (!(o instanceof Map.Entry))
            {
                return false;
            }
            Map.Entry<?, ?> e = (Map.Entry<?, ?>) o;
            V v = ConcurrentReferenceHashMap.this.get(e.getKey());
            return v != null && v.equals(e.getValue());
        }

        @Override
        public boolean remove(Object o)
        {
            if (!(o instanceof Map.Entry))
            {
                return false;
            }
            Map.Entry<?, ?> e = (Map.Entry<?, ?>) o;
            return ConcurrentReferenceHashMap.this.remove(e.getKey(), e.getValue());
        }

        @Override
        public int size()
        {
            return ConcurrentReferenceHashMap.this.size();
        }

        @Override
        public boolean isEmpty()
        {
            return ConcurrentReferenceHashMap.this.isEmpty();
        }

        @Override
        public void clear()
        {
            ConcurrentReferenceHashMap.this.clear();
        }
    }

    /* ---------------- Serialization Support -------------- */

    /**
     * Save the state of the <tt>ConcurrentReferenceHashMap</tt> instance to a stream (i.e., serialize it).
     * @param s the stream
     * @serialData the key (Object) and value (Object) for each key-value mapping, followed by a null pair.
     * The key-value mappings are emitted in no particular order.
     * @throws IOException if an i/o error occurs
     */
    private void writeObject(java.io.ObjectOutputStream s) throws IOException
    {
        s.defaultWriteObject();

        for (int k = 0; k < segments.length; ++k)
        {
            Segment<K, V> seg = segments[k];
            seg.lock();
            try
            {
                HashEntry<K, V>[] tab = seg.table;
                for (int i = 0; i < tab.length; ++i)
                {
                    for (HashEntry<K, V> e = tab[i]; e != null; e = e.next)
                    {
                        K key = e.key();
                        if (key == null)
                        {
                            // Skip GC'd keys
                            continue;
                        }

                        s.writeObject(key);
                        s.writeObject(e.value());
                    }
                }
            }
            finally
            {
                seg.unlock();
            }
        }
        s.writeObject(null);
        s.writeObject(null);
    }

    /**
     * Reconstitute the <tt>ConcurrentReferenceHashMap</tt> instance from a stream (i.e., deserialize it).
     * @param s the stream
     * @throws IOException if an i/o error occurs
     * @throws ClassNotFoundException if a class is not found
     */
    private void readObject(java.io.ObjectInputStream s) throws IOException, ClassNotFoundException
    {
        s.defaultReadObject();

        // Initialize each segment to be minimally sized, and let grow.
        for (int i = 0; i < segments.length; ++i)
        {
            segments[i].setTable(new HashEntry[1]);
        }

        // Read the keys and values, and put the mappings in the table
        for (;;)
        {
            K key = (K) s.readObject();
            V value = (V) s.readObject();
            if (key == null)
            {
                break;
            }
            put(key, value);
        }
    }
}